The variations of elastic parameters and attenuation of rock under deformation are of great importance for a number of geophysical problems. Here, we estimate elastic parameters and their evolutions during laboratory rock deformation experiments, while developing a Monte-Carlo full-waveform fitting method. The transducer-transducer one-source one-station active seismic data of dry and water-saturated samples are obtained from Zaima and Katayama (2018, https://doi.org/10.1029/2018JB016377). The synthetic seismic data are modeled using the spectral element method. We first performed a trial-and-error estimate of the boundary conditions in order to suppress its influence on waveform matching. The synthetic seismic data are generated using equivalent homogeneous models with different combinations of elastic and anelastic parameters. We then compared the synthetics with the laboratory experimental data. Based on these comparisons, we obtain the time-lapse variations of seismic velocities and attenuation of rock samples during deformation, which are then interpreted as crack development. Our simultaneous estimation of elastic and anelastic parameters allowed us to detail the dynamics prior to the rock failure.

Triaxial compression experiments were carried out on samples of fine-grained Aji granite to measure the evolution of permeability during deformation prior to failure under confining pressures of 20 and 40 MPa. During the initial stages of deformation, a small decrease in permeability was observed, due to the closure of pre-existing microcracks; permeability then increased with increasing differential stress. During deformation, permeability varied by up to two orders of magnitude, and we observed a small pressure dependence, with a larger variation observed at 20 MPa than at 40 MPa. This suggests that more cracks developed during brittle deformation under the lower confining pressure. The observed increase in permeability during our experiments was approximately proportional to inelastic volumetric strain, which corresponded to the volume of dilatant cracks. On the other hand, prior to brittle failure we observed a further increase in permeability that was greater than the inelastic volumetric strain, suggesting crack aperture opening accelerated at this stress level (>~80%).

To investigate the influence of hydration on brittle deformation of oceanic crustal rocks, we conducted triaxial deformation experiments on gabbroic rocks with various degrees of hydration. Additional experiments were performed on samples of serpentinite and serpentinized peridotite for comparison. Elastic wave velocities were measured during deformation to monitor the development of stress-induced cracks. Hydrated olivine gabbros reached a maximum differential stress of 225–350 MPa, which was considerably less than that recorded for gabbros (~450 MPa), but comparable to those for serpentinized ultramafic rocks (250–300 MPa). Elastic wave velocities of hydrated olivine gabbros were almost constant during deformation and did not show a marked decrease, even immediately prior to failure. This indicated that the deformation of hydrated olivine gabbro is not associated with the opening of the stress-induced axial cracks that are responsible for dilatancy and are commonly observed during deformation of crystalline rocks. Microstructural observations of the samples recovered after deformation showed crack damage to be highly localized to shear fracture zones with no trace of stress-induced crack opening, consistent with the absence of dilatancy. These data suggest that brittle deformation of hydrated olivine gabbro can be accommodated by the development of shear cracks in hydration minerals such as serpentine and chlorite, even when they are present in only small amounts. This leads to non-dilatant brittle deformation and a weakening of fracture strength, similar to that observed during deformation of serpentinized peridotite. Our results suggest that the brittle behavior of the oceanic crust may change considerably due to hydration.

We quantitatively evaluate transducer-transducer one-source one-station active seismic waveform data, in order to monitor time-lapse changes of elastic and anelastic structure during deformation experiments in laboratory. The experiment data of dry and water-saturated sample are provided by Zaima and Katayama (2018, https://doi.org/10.1029/2018JB016377). A transducer receiver, at the mid-point of cylindrical rock sample, is located on the antipodal position of the transducer source, emitting compressional and shear waves. Due to the extremely underdetermined nature of inverse problem, we limit the number of unknowns to be four: global P- and S- wave velocities and their corresponding anelastic attenuation factors, which can represent the micro-cracks nucleation during the loading and before the appearance of the largest crack that causes the fracture. We first performed a trial-and-error search for a realistic boundary condition in three-dimensional seismic waveform modeling using spectral-element method, in order to fit the synthetic data with the observed waveforms. We then generated synthetic data for 6000 combinations of elastic and anelastic parameters, in order to conduct Monte-Carlo waveform inversion based on the cost functions using waveform misfit and zero-lag cross-correlation. We obtained the time-lapse changes in velocity and attenuation during the deformation, which are then linked to crack development. Compared with the wet experiment, the dry experiment has a larger change in both the velocity and attenuation. However, regardless of the configuration, global seismic wave speeds rise first and then decrease during the experiments. The quality factor shows roughly the same trend.

The presence of smectite is critical for weakening the frictional strength of natural faults. The friction coefficient of smectite changes depending on water presence, chemical composition, and preferred orientation. These various factors determine the frictional properties of smectite in a complex manner, and it is difficult to understand the synergistic effects on friction. Here, we examine the synergistic effect of preferred orientation, high affinity of clays to water, and water lubrication. Highly preferentially oriented montmorillonite was prepared as self-supporting sheets, which were dried at temperatures of 70–200°C for 12 h before conducting shear experiments. The humidity-controlled double direct-shear tests of these sheets and powdered montmorillonite were conducted at room temperature under an applied normal stress from 5 to 40 MPa. No effect of drying temperature was observed for the friction coefficients of the powdered gouges, while those of the oriented sheets increased with increasing drying temperature. The slow dehydration of water in the oriented samples was confirmed by thermogravimetry-differential thermal analysis. These results indicate that the preferred orientation of smectite reduces the frictional strength by water lubrication without increasing the pore pressure. Water supplied from the ground and sea during sedimentation enhances the lubrication of oriented smectite at relatively shallow depths and approximately 200°C. The presence of oriented smectite on the subducting plate can hold more water than estimated using smectite powder, and this water in smectite may deepen the updip seismogenic zone boundary by water lubrication.